The rapid turnover of luciferase and the sensitive, noninvasive nature of its assay make this reporter gene uniquely situated for temporal gene expression studies. To determine the in vivo regulatory pattern of the Drosophila clock gene period (per), we generated transgenic strains carrying a luciferase cDNA fused to the promoter region of the per gene. This has allowed us to monitor circadian rhythms of bioluminescence from pacemaker cells within the head for several days in individual living adults. These high time-resolution experiments permitted neuronal per transcription to be accurately measured in single animals. This new technology has revealed a novel feature of per transcription and opens the door to vastly simplified experiments in general chronobiology and studies of temporally regulated transcription in a wide range of experimental systems.